1. Field of the Invention
This invention relates to the operation of fractional distillation columns, which are used on a large scale to separate the constituents of petroleum and of the products from petroleum cracking, among other purposes. This invention is particularly useful in the separation of materials which are gases at ordinary temperatures and pressures and therefore require large refrigeration capacity to maintain adequate reflux levels for proper column operation. The invention is most particularly adapted to improving the operation of demethanizers in typical ethylene production plants or natural gas conditioning plants.
2. Description of the Prior Art
Fractional distillation is a well-known process for separating mixtures of distinct chemical substances into fractions with different relative volatility. The equipment used is commonly called a column; the mixture to be separated, in the usual commercial continuous version of the process, is called a feed stream, which is separated into a most volatile overhead stream, a least volatile bottoms stream, and often into one or more intermediate volatility side streams. The general fractional distillation process is old, so that there are many distillation columns in existence which were built many years or even decades ago. Since the construction of these early columns, improved column packing materials and other operating improvements have made it possible to obtain more effective separations within the same column volume than was originally possible. More effective separation results in delivery of a vapor phase with a lower dew point to the condenser section at the top of the column.
A certain minimum level of total reflux liquid, i.e., liquid flowing down onto the top of the fractionating zone of the column, is required for proper and efficient operation of the column. Normally, this reflux liquid volume is obtained solely by condensing a suitable fraction of the vapor delivered at the top of the column. If the feed stream contains highly volatile compounds, the temperature required to condense the overhead vapors is below that of cooling water or ambient air, even when the column is operated under high pressure. In distillations of this type, which are quite common, the heat of reflux condensation must be removed by a refrigeration system.
The purity of the overhead product delivered from a distillation column depends on the quality of the column itself, including the nature and amount of packing present, and on the amount of reflux provided during operation. Obtaining a higher purity overhead product requires a colder reflux condensation temperature as well as a higher reflux volume. Thus to obtain higher purity of a high volatility overhead product from a column by conventional means, lower temperature refrigerant is usually necessary along with greater total heat removal during operation.
It is relatively inexpensive to fit an old column with new packing or to increase the reflux ratio of operation of an existing column, and both have often been done in practice to achieve greater separation efficiency than was planned or obtained when the column was originally designed. The separation efficiency of an old column can be increased so much by these means that the originally installed refrigeration capacity of the condenser section is inadequate to maintain minimum reflux volume from the lower dew point vapor delivered by the more efficient separation. This result is particularly likely when separating the lightest gases from petroleum, natural gas, or the products of petroleum cracking. It is generally much more expensive to install new refrigeration capacity than to change the reflux ratio or install new packing for an existing distillation column. As a result, actual realization of the improved efficiency theoretically achievable from existing distillation columns has been seriously impeded. Such impediment has been particularly marked in connection with demethanizer units of large scale commercial ethylene production plants.